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Description: The spread of invasive tree species has caused increasing harm to the environment. This study was motivated by the considerations that earlier studies generally ignored the role of economic factors related to the occurrence and abundance of invasive species, and empirical analyses of invasive trees on forestland have been inadequate. We assessed the impact of ecological and economic factors on the occurrence and number of invasive tree species on Alabama forest land since 1990. Count data models were used to analyze the 2004 Alabama Forest Inventory Analysis (PIA) data. The proportion of PIA plots with invasive species was 1.06 percent in 1990, 1.24 percent in 2000, and 1.35 percent in 2004. Occurrence of invasive trees on a plot depended on forest type (e.g., natural pines, planted pines, or oak-pines). Plots with planted pine were 171 percent less likely to be infested with invasive trees than otherwise similar plots. Number of invasive trees per plot was determined by a wide variety of factors, including site productivity, growing stock, stand age, ecoregion, plot proximity to metropolitan centers, ownership type, and forest management activity. The magnitude of the impact was especially large for the ownership type, with more invasive trees on private forest lands than on public lands. These findings suggest that both ecological and economic factors need to be considered in the prevention and control of invasive species invasion.